This invention relates to methods and systems for providing broadband communications to mobile users in a satellite-based communications network.
Wired terrestrial systems offer communications at high data rates, but only while the user is sitting behind a computer. As soon as the user goes to a conference room, walks outside an office building, gets into a car, or drives to a park, the connection is lost. Mobility, however, can be supported in one of two ways, namely terrestrial-based wireless networks or satellite-based communications systems.
Terrestrial-based wireless networks provide voice or data communications between a mobile user and a fixed user or to other mobile users, as well as communications for modem-equipped computers and other similar devices such as mobile facsimile machines. Existing wireless networks have not been optimized for a mix of voice, data, and video, however, despite the trend towards multimedia traffic. Several wireless and wired standards, such as asynchronous transfer mode (ATM), are being designed to optimize multimedia traffic. Wireless wide area networks (WANs) typically carry voice, whereas wireless local area networks (LANs) typically carry data. Most wireless WAN traffic operates at under 19.2 kbps. Wireless LANs that support data rates up to 10 Mbps have begun to appear, but they are limited in range to tens of meters.
A typical terrestrial-based wireless network includes a grid of service zones or cells, with each cell having a base station situated near its center. A mobile user located in a particular cell is connected to that cell""s base station through low-power radio frequency (RF) transmissions. Each base station is connected by trunk lines to other gateways, which in turn are connected by trunk lines to various other networks. Each of these cells requires costly infrastructure development and covers only a very small area. Placing a wireless base station every 200 m to provide global mobile communications is a very costly and time-consuming endeavor. In addition, the elevation angle between the user and the base station is relatively low for terrestrial-based wireless networks. At high frequencies, obstructions such as trees, buildings, signs, etc. can interfere with reliable communications.
To provide wireless service, satellite-based communications systems have been proposed which would provide world-wide fixed or low-rate (mainly voice) mobile coverage. These proposed systems typically include axe2x80x94constellation of satellites in one orbit only, such as geostationary earth orbit (GEO) only or non-geosynchronous orbit (NGSO) only. Communications satellites in geosynchronous orbit provide coverage in predetermined areas on the earth from the equator. Coverage is typically excluded from the oceans so that satellite capacity is not wasted on non-populated areas. Communications satellites in geosynchronous orbit, however, provide limited coverage at higher or lower latitudes than the Equator.
Communications satellites in non-geosynchronous orbit, such as medium earth orbit (MEO) or low earth orbit (LEO), travel relative to the Earth""s rotation and typically provide high elevation angle coverage at the higher and lower latitudes, and since they are closer to earth, propagation time delays are minimized. Non-geosynchronous communications satellites, however, waste satellite capacity over the oceans during their orbit and currently do not support broadband service to mobile users.
Data rates up to 19.2 kbps, as available from wireless WANs, will not meet future data rate needs of consumers. For example, many computer users are upgrading their wired modems to 56.6 kbps whenever possible. Such users desire a fast response from their modems even while they are away from their desks. In addition, the nature of the information being transferred is changing from short, text-based electronic mail messages to communications with embedded video clips or file attachments. Such media-rich messages consume high bandwidth and communications resources, thus requiring high data rates to allow them to be transmitted and received within a reasonable period of time.
Thus, there exists a need for a wireless communications system that provides broadband communications to mobile users. There also exists a need for an efficient satellite communications system that provides global communications service while maximizing the useful capacity of the satellites, reducing the perceived time delay, and maximizing the minimum elevation angle across latitudes.
The present invention provides a broadband satellite communications system providing global broadband network services to mobile users. The system includes a plurality of satellites each having uplink and downlink antennas for transmitting and receiving a plurality of signals utilizing a plurality of spot beams to and from a plurality of coverage areas at a predetermined range of frequencies. Each of the plurality of satellites transmits and receives the plurality of signals at one of a first plurality of data rates. The system further includes a plurality of user terminals for transmitting and receiving signals to and from the plurality of communications satellites at the predetermined range of frequencies and at one of the first plurality of data rates. Each of the user terminals have a steerable antenna for tracking relative movement of each of the user terminals with respect to each of the plurality of communications satellites and for tracking movement of each of the plurality of communications satellites so as to maintain communications with the plurality of communications satellites.
In a further aspect of the invention, a portable antenna assembly is provided that connects to an output port of an electronic device such as a computer or a telephone. The portable antenna assembly has a connector coupled to a transmission wire, which in turn is coupled to an antenna element. The antenna element sends and receives signals from a satellite. The antenna may also include a tracking device so that the direction of the antenna may be changed in response relative movement of the antenna location and the location of the satellites.
One advantage of the present invention is that power used by the satellite system may be conserved by using a routing switch that routes signals according to their desired destination. Typically, a packet switch is used to route signals. If the received beam is to be transmitted through to the same beam, the carrier frequency can be shifted and the signal returned through the same beam. These signals thus bypass the packet switch. This saves energy because the packet switch performs other operations on the signal such as demodulation, instruction reading and remodulation, each of which consumes energy. The extra functions are unnecessary if the signal is routed through the same beam.
In yet another aspect of the invention, the spot beams of two satellites may be combined on the ground to provide ubiquitous coverage over the entire land mass. If medium Earth orbit satellite orbits are used, at least two MEO satellites are in view at any one time with the preferred constellation.